util: fix crash when starting macvtap interfaces

util: fix crash when starting macvtap interfaces

This patch resolves https://bugzilla.redhat.com/show_bug.cgi?id=815270

The function virNetDevMacVLanVPortProfileRegisterCallback() takes an
arg "virtPortProfile", and was checking it for non-NULL before using
it. However, the prototype for
virNetDevMacVLanPortProfileRegisterCallback had marked that arg with
ATTRIBUTE_NONNULL(). Contrary to what one may think,
ATTRIBUTE_NONNULL() does not provide any guarantee that an arg marked
as such really is always non-null; the only effect to the code
generated by gcc, is that gcc *assumes* it is non-NULL; this results
in, for example, the check for a non-NULL value being optimized out.

(Unfortunately, this code removal only occurs when optimization is
enabled, and I am in the habit of doing local builds with optimization
off to ease debugging, so the bug didn't show up in my earlier local
testing).

In general, virPortProfile might always be NULL, so it shouldn't be
marked as ATTRIBUTE_NONNULL. One other function prototype made this
same error, so this patch fixes it as well.